(1) Field of the Invention
The present invention relates to an organic electroluminescent device with uniform driving current.
(2) Description of the Related Art
Organic electroluminescent panels use organic light emitting diodes as light sources. The organic light emitting diodes belongs to current-driving elements which change brightness with electric current flowing through the organic light emitting diode. Therefore, it is important to precisely control the electric current to flow through organic light emitting diode.
FIG. 1A shows a circuit diagram of a pixel array in a conventional active organic light emitting panel. FIG. 1B shows a plane diagram of one pixel of the pixel array in FIG. 1A. Each of pixel units 10 includes an organic light emitting diode 11, a switching transistor 12, a driving transistor 13, a data-line 14, a scan line 15, a power line 16 and a capacitor 17. Their connections have been obvious to one having ordinary skill in the art.
All driving transistors 13 of the pixel array are connected to a common display voltage source VDD with either source electrodes or drain electrodes, in order to generate a display voltage level. The source electrodes or drain electrodes, which are not connected to the display voltage source VDD, are connected to the organic light emitting diodes 11. The organic light emitting diode 11 has one end connected to the driving transistor 13, and the other end connected to the ground electrode or a reference voltage source Vss to get a reference voltage level. The difference between the display voltage level and the reference voltage level cooperates with the data voltage input from the data line 14 to effect the electric current flowing through the organic light emitting diode 11, in order to control the brightness.
Generally, the reference voltage level is constant, so the stability of the display voltage level affects the difference between the display voltage level and the reference voltage level. However, as the electric current which is applied by the display voltage source VDD goes to different pixel units by way of the power line 16, the different IR drops happen due to the factors such as materials, thickness of the power line 16, pathway of electric current, or distance from each pixel unit to the display voltage source VDD. Therefore, the practical operating voltage of the pixel unit is different from the voltage applied by the display voltage source VDD. The result are that unstable electric current occurs and flows through the organic light emitting diode 11 of each pixel unit, and that brightness is hard to control.
For improving the above disadvantage, the thickness of the metal wire such as data line, scan line or power line etc., is increased to prevent the unstable electric current. For example, the thickness of the power line 16 is increased to stabilize the electric current which flows through the source electrode or the drain electrode of the driving transistor 13.
Refer to FIG. 1C, buffer layers 131, such as silicon nitride or silicon oxide, gate oxide layer 132, dielectric layer 133, metal layer 134, protecting layer 135 and flat layer 136 etc., are disposed on the substrate 130. The metal layer 134 acts as the source/drain electrode of the transistor 13, and has a thickness reaching to 6000 Å.
Refer to FIG. 1D, the ordinate is uniformity of the display voltage (%), and the abscissa is the thickness of the metal layer (Å). In average, as the thickness of the metal layer 134 is increased from 2000 Å to 6000 Å, the uniformity of the display voltage is increased from 78% to 88%. Therefore, the electric resistance and IR drop can be reduced by increasing the thickness of the metal layer.
However, the increased thickness of the metal layer results in the following issues: the time and cost in PVD or etching process increase; high step coverage ability requirement or a flat process employed due to the ITO layer and the protecting layer 135 fabricated in the latter process; corrosion happening in the latter process due to the sidewall of the metal layer 134 covered incompletely; the structure peeling off due to larger stress.